Cancer is a major worldwide public health problem; in the United States alone, approximately 570,000 cancer-related deaths were expected in 2005. See, e.g., Jemal et al., CA Cancer J. Clin. 55(1):10-30 (2005). Many types of cancer have been described in the medical literature. Examples include cancer of the blood, bone, lung (e.g., non-small-cell lung cancer and small-cell lung cancer), colon, breast, prostate, ovary, brain, and intestine. The incidence of cancer continues to climb as the general population ages and as new forms of cancer develop. A continuing need exists for effective therapies to treat subjects with cancer.
Nucleoside analogs have been used clinically for the treatment of viral infections and certain cancers. Most nucleoside analogs are classified as anti-metabolites. After they enter the cell, nucleoside analogs are successively phosphorylated to nucleoside 5′-mono-phosphates, di-phosphates, and tri-phosphates.
The nucleoside analogs 5-azacytidine (also known as 4-amino-1-β-D-ribofuranosyl-1,3,5-triazin-2(1H)-one; National Service Center designation NSC-102816; CAS Registry Number 320-67-2; azacitidine; Aza and AZA; and currently marketed as VIDAZA®) and 2′-deoxy-5-azacytidine (also known as 5-aza-2′-deoxycytidine, decitabine, 5-aza-CdR, Dac, and DAC, and currently marketed as DACOGEN®) are DNA methyltransferase (DNMT) inhibitors that have been approved by the U.S. Food and Drug Administration for the treatment of myelodysplastic syndromes (MDS). Azacitidine and decitabine are cytidine analogs; a structural difference between these cytidine analogs and their related natural nucleosides is the presence of a nitrogen at position 5 of the cytosine ring in place of a carbon. Azacitidine may be defined as having a molecular formula of C8H12N4O5, a molecular weight of 244.21 grams per mole, and a structure as shown below. Decitabine may be defined as having a molecular formula of C8H12N4O4, a molecular weight of 228.21 grams per mole, and a structure as shown below.

After its incorporation into replicating DNA, 5-azacytidine or 5-aza-2′-deoxycytidine can form a covalent complex with DNA methyltransferases. DNA methyltransferases are responsible for de novo DNA methylation and for reproducing established methylation patterns in daughter DNA strands of replicating DNA. Inhibition of DNA methyltransferases can lead to DNA hypomethylation, thereby restoring normal functions to morphologically dysplastic, immature cells by re-expression of genes involved in normal cell cycle regulation, differentiation and death. The cytotoxic effects of cytidine analogs can cause the death of rapidly dividing cells that are no longer responsive to normal cell growth control mechanisms. 5-Azacytidine, unlike 5-aza-2′-deoxycytidine, also incorporates into RNA. The cytotoxic effects of azacitidine may result from multiple mechanisms, including inhibition of DNA, RNA and protein synthesis, incorporation into RNA and DNA, and activation of DNA damage pathways.
5-Azacytidine and 5-aza-2′-deoxycytidine have been tested in clinical trials and showed significant activity, such as, for example, in the treatment of myelodysplastic syndromes (MDS), acute myelogenous leukemia (AML), chronic myelogenous leukemia (CML), acute lymphocytic leukemia (ALL), and non Hodgkin's lymphoma (NHL). See, e.g., Aparicio et al., Curr. Opin. Invest. Drugs 3(4): 627-33 (2002). 5-Azacytidine has undergone NCI-sponsored trials for the treatment of MDS and has been approved for treating all FAB subtypes of MDS. See, e.g., Kornblith et al., J. Clin. Oncol. 20(10): 2441-2452 (2002); Silverman et al., J. Clin. Oncol. 20(10): 2429-2440 (2002). 5-Azacytidine may alter the natural course of MDS by diminishing the transformation to AML through its cytotoxic activity and its inhibition of DNA methyltransferase. In a Phase III study, 5-azacytidine administered subcutaneously significantly prolonged survival and time to AML transformation or death in subjects with higher-risk MDS. See, e.g., P. Fenaux et al., Lancet Oncol., 2009, 10(3):223-32; Silverman et al., Blood 106(11): Abstract 2526 (2005).
Other members of the class of cytidine analogs include, for example: 1-β-D-arabinofuranosylcytosine (Cytarabine or ara-C); pseudoisocytidine (psi ICR); 5-fluoro-2′-deoxycytidine (FCdR); 2′-deoxy-2′,2′-difluorocytidine (Gemcitabine); 5-aza-2′-deoxy-2′,2′-difluorocytidine; 5-aza-2′-deoxy-2′-fluorocytidine; 1-β-D-ribofuranosyl-2(1H)-pyrimidinone (Zebularine); 2′,3′-dideoxy-5-fluoro-3′-thiacytidine (Emtriva); 2′-cyclocytidine (Ancitabine); 1-β-D-arabinofuranosyl-5-azacytosine (Fazarabine or ara-AC); 6-azacytidine (6-aza-CR); 5,6-dihydro-5-azacytidine (dH-aza-CR); N4-pentyloxycarbonyl-5′-deoxy-5-fluorocytidine (Capecitabine); N4-octadecyl-cytarabine; and elaidic acid cytarabine.
5-Azacytidine and certain other cytidine analogs are approved for subcutaneous (SC) or intravenous (IV) administration to treat certain proliferative disorders. Oral dosing of cytidine analogs would be more desirable and convenient for patients and doctors, e.g., by eliminating injection-site reactions that may occur with SC administration and/or by permitting improved patient compliance. However, oral delivery of cytidine analogs has proven difficult due to combinations of chemical instability, enzymatic instability, and/or poor permeability. For example, cytidine analogs have been considered acid labile and unstable in the acidic gastric environment. Previous attempts to develop oral dosage forms of cytidine analogs have required enteric coating of the drug core to protect the active pharmaceutical ingredient (API) from what was understood and accepted to be therapeutically unacceptable hydrolysis in the stomach, such that the drug is preferably absorbed in specific regions of the lower gastrointestinal tract, such as the jejunum in the small intestine. See, e.g., Sands, et al., U.S. Patent Publication No. 2004/0162263 (application Ser. No. 10/698,983). In addition, a generally accepted belief in the art has been that water leads to detrimental hydrolytic degradation of cytidine analogs during formulation, subsequently affecting the stability of the API in the dosage form. As a result, coatings applied to the drug core for prospective oral delivery of cytidine analogs have previously been limited to organic solvent-based systems to minimize exposure of the API to water.